Mach-Zhnder (MZ)-type add-drop filters are known (see, for instance, F. Bilodeau et al., IEEE Photonics Technology Letters, Vol. 7(4), p. 388, April 1995), and are expected to find use, inter alia, in dense wavelength-division-multiplexed (WDM) optical communication systems. Such systems will typiclly require the ability to passively multiplex and de-multiplex channels at the link ends and, at least in some architectures, to add and/or drop channels at selected points on the link. These abilities can be provided by the above referred-to MZ-type devices, specifically, by such devices that comprise refractive index-gratings in both arms of an equal arm MZ-type waveguide interferometer. See U.S. Pat. No. 5,459,801 for further embodiments of add-drop filters.
Such devices, in order to provide acceptable performance, have to meet exacting requirements on, for instance, equality of arm lengths and equality of grating strengths. Furthermore, for obvious reasons, these exacting requirements will typically have to be met over a considerable range of environmental conditions, e.g., for temperatures within some defined range, and in the presence of temperature gradients, and to be substantially maintained in the presence of mechanical vibrations and other disturbances.
To date it has not been possible to meet the requirement without a "trimming" step after grating formation. See, for instance, F. Bilodeau et al. (op. cit.), which discloses an all-fiber MZ-type interferometer with photo-induced Bragg gratings, with additional length (2-3 mm) provided to do UV trimming. The Bragg gratings were formed simultaneously using a KrF excimer laser and a phase mask. Such lasers are known to typically have an output beam of high power and large cross-section but relatively low spatial and temporal coherence.
As those skilled in the art will appreciate, it is at best difficult to dispose fibers side-by-side such that the distance between the fiber cores is less than about 120 .mu.m. Furthermore, it is substantially impossible to attain equal (i.e., to within about 0.01 .lambda., where .lambda. is the operating wavelength of the device) arm lengths in a fiber MZ-type interferometer. Thus the manufacture of a fiber-based MZ-type add-drop filter would substantially always require a trimming operation, even if the two Bragg gratings were identical. Trimming, however, is highly disadvantageous, since it requires inspection of each device, and individualized exposure to UV radiation of the devices. It clearly would be highly advantageous if individualized trimming could be substantially eliminated. This however does not appear to be possible in fiber MZ-type interferometers. Furthermore, fiber-based interferometer-type devices typically are highly susceptible to mechanical disturbances, typically involving changes in polarization properties.
R. Kashyap et al., IEEE Photonics Technology Letters, Vol. 5(2), p. 191 (February 1993), disclose a MZ-type interferometer fabricated in Ge-doped planar silica. Planar waveguide MZ-type interferometers can be relatively easily manufactured with essentially equal arm lengths, due to the close dimensional control obtainable with standard photolithography and etching techniques. Nevertheless, R. Kashyap et al. (op. cit.) found imbalance in the arms that had to be compensated by trimming. See also U.S. Pat. No. 4,900,119, which discloses optical fiber devices, including a fiber MZ device, and the previously cited '801 patent, which discloses devices with refractive index gratings written directly into a coupler region.
In view of the importance of MZ-type interferometer devices, it would be highly desirable to have available such devices that can exhibit relatively stable performance even in the presence of some thermal and mechanical disturbances, and that generally can be manufactured without a trimming step, and also to have available a method of making such devices. This application discloses such devices and such a method.